CESTODA. 171 



Class Cestoda. Tapeworms. 



The Cestodes are internal parasites, ivhose life history in- 

 cludes a bladderworni (proscolex) and a tapeworm (strobila) 

 stage, the former in a Vertebrate or Invertebrate host, the 

 latter (with one exception) in the gut of a Vertebrate. In a 

 few cases the body is unsegmented, e.g., Archigetes and Caryo- 

 phyllceus, with one set of gonads ; in a few cases, e.g., Ligula, 

 there is a serial repetition of gonads without distinct segmenta- 

 tion of the body ; in niost cases, &.g.,T's,\\\d. and 'S>o\hx\oc.<i-^\\2\\i's,, 

 the body of the tapnvorm forms a chain of numerous joints or 

 proglottides, each ivith a set of gonads. Thus the class iii- 

 cludes tratisitions from unsegniented to segmented forms, but 

 the latter are imperfectly integrated. The general form op 

 the body is tape-like and bilaterally svnunelrical, with hooks, 

 grooves, or suckers ensuring attachment to the gut of the host. 

 The nervous system consists of longitudinal nerve strands and 

 anterior ganglionated commissures ; there are no special sense 

 organs. There is no alimentary system ; the parasite floating 

 in the digested food of its host absorbs soluble material bv its 

 general surface. There is no vascular nor respiratory sy stein, 

 and the body cavity is represented merely by irregular spaces. 

 Into some of these spaces there open ciliated funnels, the ends 

 (f the fine branches of longitudinal excretory tubes, which are 

 connected transversely at each joint and open terminally by one 

 or more pores. All tapeworms are hermaphrodite, and most, 

 if not all, are probably self-fertilising. The male reproductive 

 organs include diffuse testes, a vas deferens, and a protrusible 

 terminal cirrus. The female organs include a pair of ovaries, 

 yolk glands, a shell gland, a vagina by which spermatozoa 

 enter, a receptacle for storing spermatozoa, and a uterus in 

 which the ova develop. The embryo develops within another 

 host into a proscolex or bladdenvorm stage, which forms a 

 "head" or scolex. When the host of the bladderworm is 

 eaten by the final host, the scolex develops into an adult sexual 

 tapeivorm. With the conditions of endoparasitic life, tve may 

 associate the occurrence of fixing organs, the absence of sense 

 organs, the loiv though somewhat complex character of the 

 nervous system, the entire absence of a food canal, and the 

 prolific reproduction. 



life History of Tcenia solium. — This is one of the most 



